First, I got the Airport Extreme to see the printer, by checking the WAN selection. Had it always been checked, and then unchecked itself?
Then the other two computers in the house could print wirelessly, but not the main computer that had the printer on it. In a Print dialog box, the printer had an error mark by it.
But that became fixed when I scrolled the printer list in a Print dialog box, and saw the printer listed again. I checked it, and the box offered to set it up.
After that, the main computer could also print wirelessly.